A main circuit of inverters can include a direct voltage intermediate circuit and semiconductor switches connected to it, which can be FET or IGBT switches. The output stage of the inverter can be formed by a semiconductor pair connected in series between the positive and the negative voltage busbar of the intermediate circuit. The number of semiconductor pairs in the inverter can depend on the number of output phases, i.e. there is a separate output for each output phase. The semiconductor switches can be used for connecting the voltage of either the positive or the negative voltage busbar to the load connected to the output of the inverter. By connecting voltage pulses to the output the load can be provided with a specified voltage at a given time, the amplitude and frequency of which are changeable.
The semiconductor switches can be controlled by gate drivers connected to the control electrode of the semiconductor switches. Depending on the type of component employed, the control electrode can be called a gate or a base, for example. The gate drivers generate the desired current or voltage pulses for the control electrode of the semiconductor switch, and thus the switch can change over to the conducting state reliably. It can be difficult to generate the auxiliary voltage needed by the gate drivers cost-effectively since the potential of the output electrode of the power semiconductor (referred to as an upper semiconductor component herein) connected to the positive voltage busbar varies considerably as the state of the other power semiconductor changes. The output electrode is called, for example, an emitter or a source, where the name can vary according to the type of component. Positive turn-on voltage of the upper semiconductor component can be generated using a method known as a bootstrap method, in which positive turn-on voltage is produced from the positive voltage referenced to the negative voltage busbar. For example, positive turn-on voltage can be produced from voltage of a certain magnitude with respect to the negative voltage busbar of the intermediate circuit. In the bootstrap method the upper gate driver receives positive auxiliary voltage through the diode connected to the gate driver when the power semiconductor connected to the negative voltage busbar is in the conducting state. This way it is possible to generate the positive auxiliary voltage for the gate driver and used for igniting the switching component. In low-current power semiconductors, the switching component has been turned off by connecting the gate to the emitter potential of the component through the base resistance.
If the power semiconductor is intended for high currents, the gate driver should be able to generate a turn-off voltage which is negative with respect to the emitter of the semiconductor. When negative turn-off voltage is used, the component can be made to cut off the passing current quicker. Furthermore, by keeping the gate in the negative potential with respect to the emitter it is possible to prevent unintentional turn-on of the component, which could otherwise be caused by high change rates of the voltage over the component. Changes of the voltage over the component result from state changes of other power semiconductors. In the case of the IGBT, turn-on caused by voltage changes results from rapid change of the internal gate charge due to the influence of the Miller capacitance. Such unintentional turn-on lasts only for a few dozen nanoseconds during which a high current peak passes through the component, which leads to increased power loss in the power switch. To prevent this phenomenon, the gate of the component should be kept in a negative potential of at least about 5 volts with respect to the emitter when the component is off.
U.S. Pat. No. 6,147,887 discloses an inverter wherein a negative auxiliary voltage is generated for the gate driver of the power semiconductor connected to the positive voltage busbar of the intermediate circuit by the bootstrap method using negative voltage referenced to the positive current busbar and a diode connected to it. As a result, the upper power semiconductor can be switched off and kept switched off by using a gate voltage which is negative with respect to the emitter. This decreases component costs compared to other solutions because only one secondary winding of the power supply transformer is used to generate a negative direct voltage with respect to the positive voltage busbar of the intermediate circuit, and one diode per each output phase of the inverter.